1. Field of the Invention
The invention relates to a backlight unit and a liquid crystal display module (LCDM), and more particularly, to a backlight unit having less optical sheet and advantages of light weight and a thin profile and providing light of uniform brightness and an LCDM including the backlight unit.
2. Discussion of the Related Art
The liquid crystal display (LCD) devices are widely used for notebook computers, monitors, TV, and so on, because of their high contrast ratio and characteristics adequate to display moving images. The LCD devices use optical anisotropy and polarization properties of liquid crystal molecules to display images.
The LCD devices require a liquid crystal panel including first and second substrates and a liquid crystal layer interposed therebetween. An arrangement of the liquid crystal molecules in the liquid crystal layer is changed by an electric field induced in the liquid crystal panel to control light transmissivity.
Generally, an additional light source is required because the LCD panel is a non-emissive-type display device. Accordingly, a backlight unit is disposed under the LCD panel. The LCD device displays images using light produced by the backlight unit and supplied to the LCD panel. A cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp and a light emitting diode (LED) are used as a light source of the backlight unit. Among these light sources, the LEDs are widely used because of advantages in a size, power consumption and reliability.
FIG. 1 a cross-sectional view of the related art direct type liquid crystal display module (LCDM) using an LED as a light source. Referring to FIG. 1, the LCDM includes a liquid crystal panel 10, a backlight unit 20, a main frame 30, a bottom frame 50 and a top frame 40. The liquid crystal panel 10 includes first and second substrates 12 and 14 facing each other and a liquid crystal layer (not shown) therebetween. First and second polarizing plates 19a and 19b for controlling a polarization direction of light, are respectively disposed at front and rear sides of the liquid crystal panel 10.
The backlight unit 20 is disposed at a rear side of the liquid crystal panel 10. The backlight unit 20 includes an LED assembly 29 arranged along a length direction of at least one edges of the main frame 30, a reflective sheet 25 of white or silver color on the bottom frame 50, a light guide plate 23 on the reflective sheet 25 and an optical sheet 21 on the light guide plate 23. The LED assembly 29 is positioned at one side of the light guide plate 23 and includes at least one LED 29a emitting white light and an LED printed circuit board (PCB) 29b where the LED 29a is disposed. The optical sheet 21 includes a plurality of sheets.
The liquid crystal display panel 10 and the backlight unit 20 are combined using the main frame 30 that can prevent movement of the liquid crystal panel 10 and the backlight unit 20. The top frame 40 cover edges of the liquid crystal panel 10 and sides of the main frame 30, so the top frame 40 can support and protect of the edges of the liquid crystal panel 10 and sides of the main frame 30. The bottom frame 50 covers back edges of the main frame 30, so the bottom frame 50 is combined with the main frame 30 and the top frame 40 for modulation.
FIG. 2 is an enlarged cross-sectional view of an “A” portion in FIG. 1, and FIG. 3 is a cross-sectional view of an optical sheet in the related art LCDM.
Referring to FIGS. 2 and 3 with FIG. 1, the LED 29a is positioned at one side of the light guide plate 23 and arranged on the PCB 29b. The LED 29a has a fixed position and faces a side portion of the light guide plate 23 such that light emitted from the LED 29a is projected to the light guide plate 23. To provide a space for the LED assembly 29, the bottom frame 50 has a bending portion 51. Namely, the bottom frame 50 is bent upwardly and inwardly. The LED assembly 29 is fixed in the bending portion 51 using an element, for example, a double coated tape. The above backlight unit may be called as a side-view type.
The light F emitted from the LED 29a is projected to the light guide plate 23 and refracted or reflected to be projected onto the liquid crystal panel 10. The light F is processed into an uniform plane light source during passing the optical sheet 21. The optical sheet 21 includes four sheet of a first diffusion sheet 21a for diffusing light from the optical sheet 23, first and second prism sheets 21b and 21c for controlling a direction of light and a second diffusion sheet 21d. 
There is a gap G between the bending portion 51 of the bottom frame 50 and the light guide plate 23. A part of the light F from the LED 29a is directly projected onto the liquid crystal panel 10 without passing through the optical sheet 21 such that a light leakage problem is generated. The light leakage problem causes a hot spot problem on the displayed image. The hot spot portion is very much brighter than other portions. To prevent this hot spot problem, a printed pattern PA is formed at a rear side of each of the first and second diffusion sheets 21a and 21d. 
On the other hand, the LCD module is required to be thinner and lighter. In addition, a decrease of production costs is required. To meets these requirements, a decrease in a number of sheets of the optical sheet is very important point. Unfortunately, when the first and second diffusion sheets 21a and 21d are omitted, there is still the hot spot problem because the printed pattern PA is also omitted with the first and second diffusion sheets 21a and 21d. A process of forming the printed pattern on the first and second prism sheets 21b and 21c causes another problem. For example, there are damages on a prism pattern on the prism sheet during the printed pattern forming process. Alternately, when the first and second prism sheets 21b and 21c are omitted, there is a problem in light path such that brightness of the displayed image is reduced.